получить сообщение от сокета (receive a message from a socket)
Пролог (Prolog)
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
Имя (Name)
recvfrom — receive a message from a socket
Синопсис (Synopsis)
#include <sys/socket.h>
ssize_t recvfrom(int socket, void *restrict buffer, size_t length,
int flags, struct sockaddr *restrict address,
socklen_t *restrict address_len);
Описание (Description)
The recvfrom() function shall receive a message from a
connection-mode or connectionless-mode socket. It is normally
used with connectionless-mode sockets because it permits the
application to retrieve the source address of received data.
The recvfrom() function takes the following arguments:
socket Specifies the socket file descriptor.
buffer Points to the buffer where the message should be
stored.
length Specifies the length in bytes of the buffer pointed
to by the buffer argument.
flags Specifies the type of message reception. Values of
this argument are formed by logically OR'ing zero or
more of the following values:
MSG_PEEK Peeks at an incoming message. The data is
treated as unread and the next recvfrom()
or similar function shall still return
this data.
MSG_OOB Requests out-of-band data. The
significance and semantics of out-of-band
data are protocol-specific.
MSG_WAITALL On SOCK_STREAM sockets this requests that
the function block until the full amount
of data can be returned. The function may
return the smaller amount of data if the
socket is a message-based socket, if a
signal is caught, if the connection is
terminated, if MSG_PEEK was specified, or
if an error is pending for the socket.
address A null pointer, or points to a sockaddr structure in
which the sending address is to be stored. The length
and format of the address depend on the address
family of the socket.
address_len Either a null pointer, if address is a null pointer,
or a pointer to a socklen_t object which on input
specifies the length of the supplied sockaddr
structure, and on output specifies the length of the
stored address.
The recvfrom() function shall return the length of the message
written to the buffer pointed to by the buffer argument. For
message-based sockets, such as SOCK_RAW, SOCK_DGRAM, and
SOCK_SEQPACKET, the entire message shall be read in a single
operation. If a message is too long to fit in the supplied
buffer, and MSG_PEEK is not set in the flags argument, the excess
bytes shall be discarded. For stream-based sockets, such as
SOCK_STREAM, message boundaries shall be ignored. In this case,
data shall be returned to the user as soon as it becomes
available, and no data shall be discarded.
If the MSG_WAITALL flag is not set, data shall be returned only
up to the end of the first message.
Not all protocols provide the source address for messages. If the
address argument is not a null pointer and the protocol provides
the source address of messages, the source address of the
received message shall be stored in the sockaddr structure
pointed to by the address argument, and the length of this
address shall be stored in the object pointed to by the
address_len argument.
If the actual length of the address is greater than the length of
the supplied sockaddr structure, the stored address shall be
truncated.
If the address argument is not a null pointer and the protocol
does not provide the source address of messages, the value stored
in the object pointed to by address is unspecified.
If no messages are available at the socket and O_NONBLOCK is not
set on the socket's file descriptor, recvfrom() shall block until
a message arrives. If no messages are available at the socket and
O_NONBLOCK is set on the socket's file descriptor, recvfrom()
shall fail and set errno to [EAGAIN] or [EWOULDBLOCK].
Возвращаемое значение (Return value)
Upon successful completion, recvfrom() shall return the length of
the message in bytes. If no messages are available to be received
and the peer has performed an orderly shutdown, recvfrom() shall
return 0. Otherwise, the function shall return -1 and set errno
to indicate the error.
Ошибки (Error)
The recvfrom() function shall fail if:
EAGAIN or EWOULDBLOCK
The socket's file descriptor is marked O_NONBLOCK and no
data is waiting to be received; or MSG_OOB is set and no
out-of-band data is available and either the socket's file
descriptor is marked O_NONBLOCK or the socket does not
support blocking to await out-of-band data.
EBADF The socket argument is not a valid file descriptor.
ECONNRESET
A connection was forcibly closed by a peer.
EINTR A signal interrupted recvfrom() before any data was
available.
EINVAL The MSG_OOB flag is set and no out-of-band data is
available.
ENOTCONN
A receive is attempted on a connection-mode socket that is
not connected.
ENOTSOCK
The socket argument does not refer to a socket.
EOPNOTSUPP
The specified flags are not supported for this socket
type.
ETIMEDOUT
The connection timed out during connection establishment,
or due to a transmission timeout on active connection.
The recvfrom() function may fail if:
EIO An I/O error occurred while reading from or writing to the
file system.
ENOBUFS
Insufficient resources were available in the system to
perform the operation.
ENOMEM Insufficient memory was available to fulfill the request.
The following sections are informative.
Примеры (Examples)
None.
Использование в приложениях (Application usage)
The select() and poll() functions can be used to determine when
data is available to be received.
Обоснование (Rationale)
None.
Будущие направления (Future directions)
None.
Смотри также (See also)
poll(3p), pselect(3p), read(3p), recv(3p), recvmsg(3p), send(3p),
sendmsg(3p), sendto(3p), shutdown(3p), socket(3p), write(3p)
The Base Definitions volume of POSIX.1‐2017, sys_socket.h(0p)
